Static resistant initiator



OGL 26, 1965 R. w. scHNETTLr-:R 3,213,791

STATIC RESISTANT INITIATOR Filed July lO, 1964 ROBERT W. SCHNETTLER INVENTOR.

.BY may @zii/maw AGENT United States Patent G "P 3,213,791 STATIC RESISTANT INllIIATGR Robert W. Schnettler, Bearsville, NY., assgnor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware Filed duly 10, 1964, Ser. No. 381,823 8 Claims. (Cl. 102-28) This invention relates to an electric blasting cap assembly of the type utilizing a loose ignition mixture, in which the shell is metallic and the ignition plug structure is an elongated semiconductive body of which one end section is in direct electrically conductive contact at its entire side wall surface with the shell inner wall and an opposite end section is in spaced apart relationship with the said inner wall, to impart high resistance to premature ring by static electric charges and stray currents.

Electric blasting initiators, or blasting caps, comprise in general a metal shell, an ignition plug within the shell closing the shell and supporting the leg, or pin, wires of the firing circuit in fixed spaced relationship. The leg wires extend from outside the shell into and through the ignition plug and are connected at their terminal ends by a high resistance wire ordinarily referred to as the bridge wire. An ignition charge, i.e., a composition ignitable by heat developed from passage of electric current through the bridge wire, is disposed in operative contact with the bridge wire so as to be ignited. The ignition charge is in operative contact with a detonatable base charge to cause detonation of same, upon ignition. Generally, a detonatable primer charge having a heat sensitivity higher than that of the base charge is disposed intermediate the ignition composition and base charge and is adapted to be detonated responsive to heat from the bridge wire and to thereupon cause detonation of the base charge. Delay type initiators contain a slow burning, or fuse, composition intermediate the ignition composition and the primer or base charge as the case may be, and are ignitable by the burning ignition composition. The delay fuse, upon completion of its burning imparts heat to the primer to cause detonation of same with subsequent detonation of the base charge, or is in direct operative contact with the base charge only, as the case may be. Various dielectric sealing means, particularly for water-proofing the interior of the cap, are provided in the shell above the ignition plug, and around the lead wires extending from the ignition plug toward the power source.

Normally used ignition compositions, as above indicated, are highly heat sensitive and, accordingly, a discharge of high static voltage can cause ignition of the said composition and tiring of the initiator. The art is aware that such accidental rings can result from a direct discharge from a lead wire in the shell to the shell wall in the locus of the ignition composition. Of course, the caps can also be tired by direct high static discharge through the bridge wire via the lead wires. The initiator can also be red by the passage of a portion of a static charge through the bridge wire when the discharge is from shunted lead wires to shell at a point other than through the explosive charge.

There has always been a danger of premature tiring of electric blasting caps due to static discharge, and stray currents, particularly in seismic exploration work. However, the use of ammonium nitrate prill-fuel oil blasting agents, a relatively recent development in the explosives art, has sharply accentuated the problem to the extent that it is now important that improved resistance to such premature tiring be imparted to substantially all types of electric blasting caps.

For some time, commercial seismograph caps were generally iired by direct discharge in the order of 5,000

lfigl Patented Giet. 26, i965 volts through the ignition composition and by a discharge in the order of 12,000 volts by heating the bridge wire, when the charge was supplied by a 750 micromicrofarad capacitor, and the discharge was from shunted lead wires to shell.

In accordance with general practice, ammonium nitrate-fuel oil blasting agents are loaded in the borehole under pressurized ilow utilizing well known prill blowing equipment. Under such loading conditions, voltages as high as 35,000 with a capacity as high as 500 mmf. have been generated on the electric blasting cap lead wires previously set in place, as part of the initiator assembly. Inasmuch as conventional electric blasting caps of the non-static resistance type are generally red by a static discharge in the order of 2,000 volts through the ignition mixture and by a discharge in the order of 12,000 volts through the bridge wire at about mmf., it is important that the resistance of commercial electric blasting cap initiators of all types to premature tiring by static charges and stray currents be raised to higher levels.

Numerous structures have been proposed wherein one or both of the bared lead wires are connected to the cap shell by a conductive material outside the locus of the ignition composition. However, it is ditlicult with such a structure to maintain a proper balance of conductivity that will allow a discharge from both wires to the shell and still have sufficient resistance for protection against the lower voltage currents (stray currents) which attend many commercial blasting operations. Additionally, it has been found that in some instances the static resistance of this type of structure diminishes with storage. Various high static resistance seismic type blasting cap structures, each containing a semi-conductive plug assembly, are disclosed and claimed in U.S. Patents 2,658,451 and 2,802,422, issued November 10, 1953 and August 13, 1957, respectively, to Charles F. Horne, in U.S. Patent 2,802,421, issued August 13, 1957 to Charles F. Horne and Edward L. Ratner, and in U.S. 2,974,590, issued March 14, 1961 to Edward L. Ramer, the said patents being assigned to Hercules Powder Company.

rIlhis invention is concerned with electric blasting caps characterized iby improved resistance to premature tiring yby high static charges and stray currents, and especially well applied as initiators in connection with ammonium nitrate-fuel oil blasting agents loaded under pressurized ow conditions.

In accord-ance with this invention, an electric blasting cap exhibiting high resistance to premature ring by static electric charges and stray cur-rents is provided which comprises a substantially cylindrical metallic shell, closed at one end; 1a dielectric body within said shell, and spaced from the said closed end, as a closure therefor; an elongated semiconductive plug within, and coaxial with, said shell intermediate said closed end and said dielectric body, said :plug containing substantially cylindrical fend sections of different cross sectional diameters and disposed with its larger diameter end section adjacent said dielectric plug and its small-er diameter end section intermediate said larger diameter `end section and said closed shell end, and stepped inwardly from the larger diameter end section to the smaller diameter end section, iat least initially, along a conical surface at an angle alpha, described hereinafter; said semiconductive body being disposed at the entire side wall surface of Kits larger diameter end section in direct, and electrically conductive, contact wit-h the inner wall of said shell, and `al-ong the remainder of its Alength in spaced a-part relationship with the shell inner wall to #form an annulus therewith; a pair of electrical conductor wires extending longitudinally, and spaced apart, into said shell through said dielectric ybody and said semiconiductive plug and terminating intermediate said closed shell .end `and t'he said semicondiuctive plug; an electrical resist- 3 ance wire connecting the terminal ends of said conductors; an ignition composition, within said shell, intermediate said semiconductive plug and said closed shell end in direct contact with said resistance Wire and ignitable in response to heat developed by passage of electric current through said resistance wire via said conductors; a detonat-able explosive charge within said shell intermediate said ignition composition and said closed end and in .operative contact 'lwith said ignition composition to detonate in response to ignition of said ignition composition upon passage of electric current through ysaid resistance wire; .said semiconductive plug consisting essentially of -a nonconductive material having a cold ow point above 150 F. .and containing particulate aluminum uniformly dispersed therethrough and characterized by an average particle size of from 1 to l15 microns (Fisher suby sieve size) in ian amount of from 50 to 60 weight percent; and the said angle alpha being .at lleast 2 and not m-ore than 60 and formed by the intersect-ion lof a straight line extending from the longitudinal axis of said semiconductive ig- .nition plug along the above said conical surtace and -away from said plu-g, and a line along the wall of said large diameter end section parallel to the longitudinal axis there-of.

In accordance with a now preferred embodiment of the invention, the inwardly stepped section of the semiconductive ignition plug is in two stages, the second stage being at substantially 90 to the longitudinal axis of the plug. A dielectric sleeve is disposed coaxially within the shell around the resistance wire and terminal ends of the conductor wires in abutting relationship with the face of the second :stepped p-lug portion.

In a prefenred form, the assembly of .the invention is an electric delay blasting cap containing a convention-a1 delay type fuse intermediate the ignition composition and the .above said detonata'ble explosive, the above sai-d detonataible explosive in most instan-ces constituting a primer type composition detonatable in response to heat developed lby burning of the delay Ifuse, together with a base charge in the closed end of the shell in direct contact with .the primer and detonatable in response to detonation of the primer. In another iorm, the assembly of the invention is an instantaneous electric blasting cap containing .the said primer composition in direct contact with the ignition composition .and detonatable in response to heat developed -by lburning of the ignition composition and when desired, in conjunction with a base Charge in the closed end of the shell and in contact with the primer, the latter 4det-onatable in response to detonation of the primer.

The invention is illustrated with reference to the drawings, of which FIG. 1 is a front elevational view of an electric blasting cap of the instantaneous type containing a semiconductive plug assembly stepped in a single stage; FIG. 2 is an elevational view the same as that of FIG. `1 except that the electric blasting cap is of the -delay type; FIG. 3 is a perspective, more detailed, view of the semiconductive plu-g element of FIGS. 1 and 2; FIG. 4 is a perspective view of a semicond'uctive pl-ug element of the invention, stepped in .two stages and thus adapted to receive a sleeve in abutting relationship therewith; -and FIG. 5 is a sectional view conresponding to a portion of FIG. 2 but containing the semiconductive plug assembly of FIG. 4 in lieu of that of FIG. 2 in combination with a dielectric sleeve element abutting thereto. All numbers shown in FIGS. 1 to 5 correspond to like numbers to designate the same element-s and are primed in numerous instances to identify the particular element with reference -to a particular gure.

With reference to FIG. 1, shell of blasting cap 11 is substantially cylindrical and metalli-c and closed at its bottom end 12, and contains a dielectric plug 13 spaced from closed end 12 as a closure for the shell, and elongated semiconductive ignition plug 14 intermediate closed end 12 and dielectric plug 13. Ignition plug 14 is coaxial with shell 10 and contains substantially cylindrical end sections 16 and 17 of diiferent cross sectional diameters, and is stepped inwardly from larger diameter end section 16 to smaller diameter end section 17 along the surface of remaining, and conically shaped, section 15 of its length. Larger diameter end section 16 is disposed at its entire side Wall surface in direct, and electrically conductive, contact with the inner wall of shell 10 to close same, and the remainder of -the length of plug 14, extending toward cl-osed shell end 12, is disposed in spaced apart relationship with the shell inner wall to form an annulus 18.

Electrical conductor wires 19 covered by insulation 21, outside shell 10, extending longitudinally, and spaced apart into shell 10 and through dielectric plug 13 and ignition plug 14 terminating in shell 10 intermediate ignition plug 14 land closed end 12. Conductors 19 exten-ding through dielectric plug 13 can be covered by insulation 21 when desired, except for a short section of length adjacent plug 14 which is ba-re to afford a water-tight seal between wires 19 and dielectric plug 13. In all events, those lengths of conductor wires 19 extending through ignition plug 14 are bare, i.e., uninsulated. Resistance, or bridge, wire 22 in shell 10 connects 4the terminal ends of conductors 19.

Ignition composition 23 is a loose ignition powder :such as lead-selenium and is disposed intermediate ignition plug 14 and closed shell end 12 in direct Contact with bridge wire 22, and is ignitable in Iresponse .to heat developed by passage 'of electric current through 'bridge wire 22 via conductors 19. Detonatable charge 24, intermediate closed end 12 and ignition composition 23, such as diazodinitrophenol, is a .suitable primer in direct contact with ignition composition 23 and detonatable in response to heat developed by ignition of composition 23. VBase charge 26 is any suitable high explosive such as pentaerythritol tetranitrate (PETN) detonatable in response to detonation of primer charge 24.

Although a single dielectric plug 13 can serve as a suitable waterproofing and sealing plug to close shell 10, it is generally supplemented in that regard by an additional dielectric material, e.g., sealing and waterproofing plug closure 27, in which event, plug 13 is disposed intermediate ignition plug 14 and the .said plug 27. Plug 13 in conjunction with plug 27 is generally an asphaltic material with plug 27 being advantageously a seal of sulfur or plastic such as ethyl cellulose.

With reference to FIG, 2, blasting cap assembly 9 is the same as assembly 11 of FIG. 1 except for the presence of .a delay fuse element intermediate, and in direct contact with, each of ignition composition 23 and primer 24', the delay element consisting of a longitudinally perforated metal carrier 20, e.g., lead, -containing a suitable fuse composition 25 in the perforation, e.g., barium peroxidetellurium, which is ignitable in response to heat developed by burning of composition 23. Primer 24' is detonatable in response to heat developed upon completion of burning of the fuse composition 25. The selecti-on of the length and lthe composition of the delay fuse 25 determines the time, or delay, intermediate the burning of the ignition composition and detonation of the primer 24', thus adapting the assembly 9 4to delay firing, which is, of course, well k-own in `the art.

With reference to FIG. 3, a perspective view of the semiconductive ignition plug assembly 14 and 14 of FIGS. 1 and 2, respectively, semiconductive plug 14 is elongated with substantially cylindrical end sections and is tape-red inwardly f-rom cylindrical end .section 16", i.e., toward its longitudinal axis, to cylindrical end section 17" of reduced or stepped down cross -sectional diameter to form the above described annulus with the shell inner wall entirely along the remaining sections 15 and 17 of its length. Angle alpha, discussed hereinabove, is, in this embodiment, generally within the range of from about 10 to 20.

With reference to FIG. 4 is shown, in perspective, -semiconductive ignition plug 30, which is exactly the same as plug 14 of FIG. 3 except that the step-down is effected in two stages. In the practice of this embodiment, the angle alpha is often within the range of about 2 to 10.

The embodiment of FIG. 4 provides a flat surface, or face 31, which serves as a stop for the dielectric sleeve element, when utilized.

End section 17a and semiconductive plug section 15a of FIG. 4 are similar -to elements 17 and 15" of FIG. 3, respectively, but differ therefrom only in respect of their 4relationship to the -second stage of the step-down of the ignition plug 30.

FIG. 5 is illustrative of a plug 30 of FIG. 4 with the sleeve element disposed in shell in conjunction with cylindrical dielectric sleeve 28 coaxially disposed in shell 10 around loose ignition mixture 23".

The semiconductive ignition plug lof the invention can be fabricated from .any suitable known non-conductor material having a cold flow above 150 F. Preferably, .the non-conductor material adheres to the bare conductor wire extending therethrough sufliciently to hold the conductor wires in place under approximately a 16-lb. pullout test. However, such contact between .the semiconductive plug and conductor wire is not required inasmuch as the requisite resistance to pull-out can be accomplished by adhesion of the conductor wires to a dielectric closure material above described and suitable for that purpose.

In accordance with a preferred embodiment, the nonconductor material is a candelilla wax composition. Other waxes include montan, and carnauba and synthetic waxes such as those manufactured under the trade names Ceramid, Acrawax C, Flexo Wax and Bareco Wax. Various mixtures of these waxes may also be employed. Of this group Acrawax is preferred.

Instead of wax, however, rubber or rubber-like materials, resinous materials, sulfur and equivalent materials may be employed. Since it is desirable that the body of semi-conductive material be molded about the lead wires, readily moldable non-conductive materials are preferred.

It is required in the practice of the invention that the conductive element of the semiconductive plug be particulate aluminum and that it be characterized by an average particle size within the range of from 1 to 15 microns as determined by the Fisher sub-sieve sizer, an average size of from 4 to 7 microns being generally preferred. The particulate aluminum can be granular or spherical.

The content of dispersed particulate aluminum in the non-conductor of the plug assembly must be within a limited range, the content in a given instance being dependent to a large extent upon the particular non-conductor material. However, the content of particulate aluminum in the semiconductive plug will generally be in the range of from 50 to 60 Weight percent.

It is an important feature of the invention that the blasting cap assembly, by virtue of the semiconductive ignition plug structure, provides a highly desirable balance of protection against firing by both stray currents and static discharge, as discussed above. The ignition plug breakdown is balanced between a minimum requirement for stray current protection and a maximum below the breakdown of the loose ignition composition. This balance is controlled by the particle size of the particulate aluminum, as Well as the amount thereof. Thus, if the average Fisher sub-sieve particle size exceeds microns, the balance is impaired. Aluminum particle size less than one micron (F.s.s.s.) is generally not desirable from the standpoint of practicability, especially when considering preparation of the dispersion.

In a now preferred practice, the ratio of the diameter of the larger to smaller cylinder end sections of the semiconductive plug is above 1.16:l and is generally not greater than about 1.311, the minimum difference between the diameters of the large and small end sections being about 0.03". Such ratios above 1.3:1 are generally not feasible from the standpoint of practicability. If the said ratio is less than 1.l6:1 or if the difference between the diameters is less than 0.030", the ignition composition in the annular space may be ignited by static discharge inasmuch as in such a small annulus, the degree of compression of the ignition composition may be such as to increase the density and thus, the conductivity of the composition to propagate static discharge.

The ratio of the length of the large diameter end section to the remaining length of the ignition plug, i.e., the stepped portion plus the smaller diameter end section, is most advantageously within the range of 0.7:1 to 4:1. Generally, it is necessary that the length of the small diameter end section be at least 0.090 to insure an annular volume sufciently large to prevent compression of the ignition composition and accompanying low resistance, which would propagate static discharge. The length of the large diameter end section is at least 0.20ll inasmuch as a shorter length may provide an insuihcent amount of side wall surface to conduct the static discharge to the shell wall, thus causing it to dissipate in the locus of the ignition composition.

Angle alpha as defined above must in all cases be at least 2 and not greater than 60. However, in all instances, the dimensional requirements of the two cylindrical end sections of the semiconductive ignition plug must also be met. This means that all angles in the above said range of 2 to 60 are not necessarily applicable in the given instance. For example, in the embodiment of FIG. 3 alpha can be as small as say, 4 to 5 and up to as high as 60 whereas in the embodiment of FIG. 4, alpha is generally not more than about l0 to 15, being as low as 2 in many instances.

It is often desirable to incorporate a sleeve element in the area of the ignition composition for control of the ignition composition volume and density, thus lessening the possibility that the voltage breakdown of the ignition composition will become lower than that of the semiconductive ignition plug. By use of a plug of the type of FIG. 4 in conjunction with a dielectric sleeve as illustrated with reference to FIG. 5, the semiconductive plug is stopped upon being inserted into the shell, thereby preventing any undue compression of the ignition composition that might possibly have occurred without the stopplng actlon.

Preferred, and exemplary, ignition compositions are lead-selenium (72-28)/leadtellurium (62-38), 50/ 50, red lead/manganese boride (-35), and lead selenium (72-28)/lead oxide-boron (97-3), 75/25, and various known additives, for example, from 1-2 percent Snow Floss. In all events, the ignition compositions employed are characterized by a minimum voltage breakdown greater than the maximum voltage breakdown of the semiconductive plug, say in the order of 1,000 volts or higher.

The base charges may be formed from any secondary detonative explosive such as pentaerythritol tetranitrate, cyclonite, tetryl, trinitrotoluene, and the like, and may be 'cast or pelleted as well as pressed when the nature of the explosive permits. When a priming charge is employed, any primary explosive or mixture can be used such as diazodinitrophenol, diazodinitrophenol-potassium chlorate, lea-d azide, lead styphnate, and mercury fulminate.

Preferred resistance wire alloys include those of nickel and chromium such as manufactured under the trade names of Tophet C and Ohmax. Diameters of the resistance wire are generally in the range of 0.001 to 0.002 to assure a proper balance between the cap firing characteristics and protection against ring by passage of static electricity through the bridge wire.

The cap shell is formed of any suitable metal, preferably brass, copper, bronze, ferrous metals, and aluminum. The lead Wires may be made of any of the conventional materials such as copper, iron and the like.

The invention is further illustrated with reference to the following examples:

EXAMPLE 1 A series of static and voltage breakdown tests was made utilizing a delay blasting cap in each instance characterized by the following specifications:

Shell-0261" LD., bronze, x 2.52 in length Base lcharge-0.40 gm. PETN/ graphite, 98/2, pressed at 3500 p.s.i.

Primer-0.30 gm. diazodinitrophenol charged into gilding metal capsule 0.484 in length x 0.245 O.D. Delay fuse-Lead tube with BaOZ/Se/Pb-Sn (S5-15),

60/20/20, 0.21 in length Dielectric sleeve-0.30 long x 0.250 O.D. x 0.238 LD. Ignition composition-Pb/ Se/ diazodinitrophenol Snow Floss 1. 70.9/27.5/0.55/1.05 Dielectric seal-Ashphaltic waterproofing and topped with sulfur The particulate aluminum uniformly dispersed throughout the non-conductive plug was of the size and in the proportions shown, and the total static charge applied was ve successive discharges of 35 kv. each supplied by a 750 mmf. capacitor from shunted leads to the cap shell. In carrying out the voltage breakdown tests, voltage was gradually increased from the cap shunted leads to the cap shell until a conductance of milliamps was realized.

In each instance, the conductive plug was 1/2 in length, the large diameter end section was 0.256 in diameter by 0.25 in length, the small diameter end section was 0.200" in diameter by 0.150 in length, and the plug was a smooth taper, i.e., single stepped, from the large diameter end section to the small diameter end section at an angle alpha (described hereinabove) of The non-conductive material was 4.5 percent mica, 0.5 percent Maraspherse (a viscosity controlling agent) and the rest, candelilla wax.

* Fisher sub-sieve average particle size.

This example illustrates the importance of aluminum particle size in maintaining control of the voltage breakdown of the plug. With the coarse aluminum, 15-20 microns, F.s.s.s., the minimum voltage breakdown was too low to provide resistance to premature tiring by stray currents, and the static protection was poor. The weight percent aluminum was low, 46 percent, in an attempt to raise the minimum voltage breakdown, but still the minimum breakdown was low. If the aluminum content were further lowered, the resistance to static discharge would have been even less. In eect, with the coarse aluminum, the semiconductive plug cannot satisfy both high minimum voltage breakdown and a high degree of static protection. On the other hand, using aluminum `of 10-25 microns F.s.s.s., which is within the scope of the invention, a desirable balance of both higher voltage breakdown and static protection are achieved.

EXAMPLE 2 `A series of static resistance tests was carried out as described in Example 1 and utilizing the same electric 1A high purified state of siliceous skeletons of microscopic aquatic plants.

blasting cap except that (l) there was no dielectric sleeve element, (2) the ratio of diameters of the end sections of the conductive plug was varied and (3) the semiconductive plug composition was Al/candelilla wax/ mica/ Maraspherse, 57/ 38/ 4.5/ 0.5, the particulate aluminum being spherical and having an average particle size of 10-15 microns F.s.s.s. One group of the series involved tests utilizing a single cylindrical semiconductive plug, i.e., the diameter ratio being 1:1. In another group, the ratio of the larger to the smaller of the diameters was 1.12: l, which is outside the range of those utilized in the practice of the invention, and in a third group, the semiconductive plug had a diameter ratio of 1.28:1, which is within the scope of the invention. The results of the tests are summarized in the following tabulation:

vThis example illustrates the elect of the ratios of the large diameter section to small diameter section. Outside the range, 1.16:l-1.30:1, the semiconductive plugs tested gave yunsatisfactory static protection.

EXAMPLE 3 A series of static discharge and voltage breakdown tests was made in accordance with the procedure of Example 1 utilizing in each instance the same electric blasting cap except (1) there was no sleeve element and the Snow Floss content tof the ignition composition was 2 percent and (2) the semiconductive plug composition was Al/ candelilla wlax/mica/Maraspherse, 52/43/4.5/ 0.5, the particulate aluminum being granular and of 5-7 microns F.s.s.s. The results of these tests are shown in the following tabulation:

Table 3 No. Minimum Statie Voltage Shot Failed Tested Voltage Breakdown This example particularly illustrates the high degree of balance obtained between high static resistance and protection yagainst stray currents obtained in the practice of the invention.

Although the invention has been specifically illustrated in terms of a single stage and a double stage step-down of the semiconductive plug assembly of the invention, it is t-o be understood that it is within the scope of the invention to effect the step-down in any suitable number of stages.

As will be evident to those skilled in the art, various modifications can be made or followed, in the light of the foregoing disclosure and discussion, without departing from the spirit or scope of the disclosure or from the scope of the claims.

What I claim and desire to protect by Letters Patent 1. An electric blasting cap assembly which comprises a substantially cylindrical metallic shell, closed at one end; a dielectric body within said shell, and spaced from the said closed end, as a closure therefor; an elongated semiconductive plug within, and coaxial with, said shell intermediate said closed end and said dielectric body, said plug containing substantially cylindrical end sections of different cross sectional diameters and disposed with its larger diameter end section adjacent said dielectric plug and its smaller diameter end section intermediate said larger diameter end section and said closed shell end, and stepped inwardly from the larger diameter end section to the smaller diameter end section, in at least a first stage, along a conical surface at an angle alpha, described hereinafter; said semiconductive body being disposed at the entire side wall surface of its larger diameter end section in direct, and electrically conductive, contact with the inner wall of said shell, and along the remainder of its length in spaced apart relationship with the shell inner wall to form an annulus therewith; a pair of electrical conductor wires extending longitudinally, and spaced apart, into said shell through said dielectric body and said semiconductive plug and terminating intermediate said closed shell end and the said semiconductive plug; an electrical resistance wire connecting the terminal ends of said conductors; an ignition composition, within said shell, intermediate said semiconductive plug and said closed shell end in direct contact with said resistance wire and ignitable in response to heat developed by passage of electric current through said lresistance wire via said conductors; ya detonatable explosive charge within said shell intermediate said ignition composition and said closed end and in operative contact with said ignition composition to detonate in response to ignition of said ignition composition upon passage of electric current through said resistance wire; said semiconductive plug consisting essentially of a non-conductive material having a cold ow point above 150 F. and containing particulate Ialuminum uniformly dispersed therethrough and characterized by an average particle size of from 1 to 15 microns (Fisher sub sieve size) in an amount of from 50 to 60 weight percent; and the said angle alpha being at least 2 and not more than 60 and formed by the intersection of a straight line extending from the longitudinal axis of said semiconductive ignition plug along the above said conical surface and away from said plug, and a line along the wall of said large diameter end section parallel to the longitudinal axis thereof.

2. A blasting ca-p assembly of claim 1 wherein said semiconductive plug is stepped in a single stage along the surface of a right cone and wherein the angle alpha is fr-om 4 to 60.

3. A blasting cap assembly of claim 1 wherein the said average particle size of said aluminum is from 4 to 7 microns.

4. A blasting cap assembly of claim 1 wherein the ratio of the length of said larger diameter end section of `said semiconductive plug to the length of the remainder of said semiconductive plug is Within the range of from 0.7:1 to 4:1, the ratio 4of the cross sectional diameter of said larger diameter end section to the cross sectional diameter of said smaller diameter end section is from 1.16:l to 130:1; the minimum length of the smaller diameter end section is 0.090 inch; the minimum length of the larger diameter end section is 0.20 inch; and the minimum difference between the cross sectional diameters of the above said end sections is 0.030 inch.

5. A blasting cap assembly of claim 1 wherein the said non-conductive component of said semiconductive plug comprises a wax.

6. An electric blasting cap assembly which comprises a substantially cylindrical metallic shell, closed at one end; a dielectric body within said shell, and spaced from the said closed end, as a closure therefor; an elongated semiconductive plug within, and coaxial with, said shell intermediate said closed end and said dielectric body,

said plug containing substantially cylindrical end sections of different cross sectional diameters and disposed with its larger diameter end section adjacent said dielectric plug and its smaller diameter end section intermediate said larger diameter end section and said closed she-ll end, and stepped inwardly from the larger diameter end section to the smaller diameter end section, in a first stage along a conical surface at an angle a, described hereinafter, and then in a second stage at substantially to the longitudinal axis of said shell; said semiconductive body being disposed at the entire side wall surface of its larger diameter end section in direct, and electrically conductive, contact with the inner wall of said shell, and along the remainder of its length in spaced apart relationship with the shell inner wall to form an annulus therewith; a pair of electrical conductor wires extending longitudinally, and spaced apart, into said shell through said dielectric body and said semiconductive plug and terminating intermediate said closed shell end and the said semiconductive plug; an electrical resistance wire connecting the terminal ends of said conductors; an ignition composition, within said shell, intermediate said semiconductive plug and said closed Shell end in direct contact with said resistance wire and ignitable in response to heat developed by passage of electric current through said resistance wire via said conductors; a detonatable explosive charge within said shell intermediate said ignition composition and said closed end and in operative contact with said ignition composition to detonate in response to ignition of said ignition composition upon passage of electric current through said resistance wire; said semiconductive plug consisting essentially of a nonconductive material having a cold ow point above F. and containing particulate aluminum uniformly dispersed therethrough and characterized by an average particle size of from 1 to 15 microns (Fisher sub sieve size) in an amount of from 50 to 60 weight percent; and the said angle a being at least 2 and not more than 60 and formed by the intersection of a straight line extending from the longitudinal axis of said semiconductive ignition plug along the above said conical surface and away from said plug, and a line along the wall of said large diameter end section parallel to the longitudinal axis thereof.

7. An electric blasting cap of claim 6 containing a dielectric sleeve coaxially disposed in said shell around said resistance wire and abutting against the stepped portion of said semiconductive plug disposed at 90 as described.

8. An electric blasting cap assembly which comprises a substantially cylindrical metallic shell, closed at one end; a dielectric body within said shell, and spaced from the said closed end, as a closure therefor; an elongated semiconductive plug within, and coaxial with, said shell intermediate said closed end and said dielectric body, said plug containing substantially cylindrical end sections of different cross sectional diameters and disposed with its larger diameter end section adjacent said dielectric plug and its smaller diameter end section intermediate said larger diameter end section and said closed shell end, and stepped inwardly from the larger diameter end section to the smaller diameter end section along the surface of a right cone at an angle a of from 2 to 15 further described hereinafter, and then at substantially 90 toward the longitudinal axis of said shell; said semiconductive body being disposed at the entire side wall surface of its larger diameter end section in direct, and electrically conductive, contact with the inner wall of said shell, and along the remainder of its length in spaced apart relationship with the shell inner wall to form an annulus therewith; a pair of electrical conductor wires extending longitudinally, and spaced apart, into said shell through said dielectric body and said semiconductive plug and terminating intermediate said closed shell end and the said semiconductive plug; an

ll electrical resistance wire connecting the terminal ends of said conductors; an ignition composition, within said shell, intermediate said semiconductive plug and said closed -shell end in direct contact with said resistance wire and ignitable in response to heat developed by passage of electric current through said resistance wire via said conductors; a detonatable explosive charge Within said shell intermediate said ignition composition and said closed end and in operative contact with said ignition composition to detonate in response to ignition of said ignition composition upon passage of electric current through said resistance wire; said `semiconductive plug consisting essentially of a nonconductive material having a cold flow point above 150 F. and containing particulate aluminum uniformly dispersed therethrough and characterized by an average particle size of from l to l5 microns (Fisher sub sieve size) in an amount of from 50 to 60 weight percent; and the said angle a being formed by the intersection of a straight line extending from the longitudinal axis of said semiconductive ignition plug along the above said conical surface and away from said plug, and a line Ialong the Wall of said large diameter end section parallel to the longitudinal axis thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,963,971 l2/60 Horne 102-28 2,970,047 l/61 Ciccone 149-38 2,974,590 3/61 Ramer 102-28 3,002,458 10/61 Haas 102-28 BENJAMIN A. BORCHELT, Primary Examiner. 

1. AN ELECTRIC BLASTING CAP ASSEMBLY WHICH COMPRISES A SUBSTANTIALLY CYCLINDRICAL METALLIC SHELL, CLOSED AT ONE END; A DIELECTRIC BODY WITHIN SAID SHELL, AND SPACED FROM THE SAID CLOSED END, AS A CLOSURE THEREFOR; AN ELONGATED SEMICONDUCTIVE PLUG WITHIN, AND COAXIAL WITH, SAID SHELL INTERMEDIATE SAID CLOSED END AND SAID DIELECTRIC BODY, SAID PLUG CONTAINING SUBSTANTIALLY CYLINDRICAL END SECTIONS OF DIFFERENT CROSS SECTIONAL DIAMETERS AND DISPOSED WITH ITS LARGER DIAMETER END SECTION ADJACENT SAID DIELECTRIC PLUG AND ITS SMALLER DIAMETER END SECTION INTERMEDIATE SAID LARGER DIAMETER END SECTION AND SAID CLOSED SHELL END, AND STEPPED INWARDLY FROM THE LARGER DIAMETER END SECTION TO THE SMALLER DIAMETER END SECTION, IN AT LEAST A FIRST STAGE, ALONG A CONICAL SURFACE AT AN ANGLE ALPHA, DESCRIBED HEREINAFTER; SAID SEMICONDUCTIVE BODY BEING DISPOSED AT THE ENTIRE SIDE WALL SURFACE OF ITS LARGER DIAMETER END SECTION IN DIRECT, AND ELECTRICALLY CONDUCTIVE, CONTACT WITH THE INNER WALL OF SAID SHELL, AND ALONG THE REMAINDER OF ITS LENGTH IN SPACED APART RELATIONSHIP WITH THE SHELL INNER WALL TO FORM AN ANNULUS THEREWITH; A PAIR OF ELECTRICAL CONDUCTOR WIRES EXTENDING LONGITUDINALLY, AND SPACED APART, INTO SAID SHELL THROUGH SAID DIELECTRIC BODY AND SAID SEMICONDUCTIVE PLUG AND TERMINATING INTERMEDIATE SAID CLOSED SHELL END AND THE SAID SEMICONDUCTIVE PLUG; AN ELECTRICAL RESISTANCE WIRE CONNECTING THE TERMINAL ENDS OF SAID CONDUCTORS; AN IGNITION COMPOSITION, WITHIN SAID SHELL, INTERMEDIATE SAID SEMICONDUCTIVE PLUG AND SAID CLOSED SHELL END IN DIRECT 